1. Field of the Invention
This invention relates generally to a photographic exposure control system and, more particulary, to a photographic exposure control system for adjusting the follow focus aperture in which a flash is fired as a function of a pre-exposure ambient scene light measurement.
2. Description of the Prior Art
In the photographic art, exposure control systems embodying scanning shutter blade elements which operate to vary exposure areas with time during the exposure interval are well known as indicated by U.S. Pat. No. 3,942,183, entitled "Camera With Pivoting Blades", by George D. Whiteside, issued Mar. 2, 1976, in common assignment herewith. Such scanning shutter blade mechanisms generally include a pair of counter-reciprocating blade elements each having a primary aperture that traverses the optical axis of the camera during the exposure interval. The primary apertures are shaped so that upon overlying one another during countermovement of the blades, there is defined an effective exposure aperture value which increases to a maximum value in a determinate period of time.
Exposure control is provided by a pair of secondary photocell apertures in respective shutter blade elements which admit scene light to a photoresponsive element in correspondence with the scene light admitted to the focal plane during shutter blade movement through an exposure cycle. The output from the photoresponsive element is directed to an integrator circuit which triggers upon reaching an integration level corresponding to a desired exposure value to terminate the exposure interval by returning the shutter blade elements back to their initial scene light blocking position.
Shutter mechanisms of the aforementioned type are often arranged to operate in synchronism with a source of artificial illumination such as an electronic flash or strobe. The shutter blade mechanism and strobe are generally arranged to operate in either a high ambient scene light intensity mode of operation where no artificial illumination is provided or a low ambient scene light intensity mode of operation where the strobe is fired to provide a source of artificial illumination. Under conditions of low ambient scene light intensity where the strobe is expected to be fired, the scanning shutter blade mechanism may be stopped at an aperture value corresponding to the camera-to-subject distance as determined by focusing the objective lens. Systems of the aforementioned type are generally referred to as "follow focus" systems and the maximum aperture to which the scanning shutter blade elements are allowed to open is controlled by the rangefinding or focusing system of the camera in a manner as is more fully disclosed in U.S. Pat. No. 3,977,014, entitled "Follow Focus Exposure Control System With Improved Uniform Trim Control", by Philip R. Norris, issued Aug. 24, 1976, in common assignment herewith.
Such a system need not actually stop the opening shutter blade movement at a maximum effective exposure aperture as determined by the rangefinding or focusing system of the camera but may, instead, vary the time at which the strobe light is fired during the opening shutter blade movement as determined in correspondence with the rangefinding or focusing system of the camera. Since the duration of the strobe light is short in comparison to the time required for the shutter blade mechanism to scan from its scene light blocking position to its maximum exposure aperture defining position, the aperture defined by the shutter blade elements at the instant of strobe firing for practical purposes constitutes the effective aperture by which the exposure occurs under conditions of low ambient scene light intensity. Such a system is more fully described in U.S. Pat. No. 3,478,660, entitled "Photographic Apparatus With Flash Exposure Control System", by Edwin H. Land, issued Nov. 18, 1969, in common assignment herewith. Range responsive flash firing systems of the aforementioned type can also be utilized in conjunction with sonar rangefinding devices as is more fully described in U.S. Pat. No. 4,167,316, entitled "Sonar Controlled Lens Focus Apparatus", by Bruce K. Johnson et al., issued Sept. 11, 1979, in common assignment herewith.
Exposure in artificial illumination control systems of the aforementioned type are generally intended to provide a source of artificial illumination only during the low ambient scene light intensity mode of operation. However, there may also be conditions during high ambient scene light intensity modes of operation wherein it becomes desirable to provide artificial illumination to fill in the photographic subject against a brightly back-lit scene as is more fully disclosed in U.S. Pat. No. 4,023,187, entitled "Exposure Control System With Fill Flash Race Condition", by Edwin K. Shenk, issued May 10, 1977, in common assignment herewith. The aforementioned artificial illumination control system operates under conditions of low ambient scene light intensity to fire the strobe light at a predetermined time period subsequent to the initiation of the exposure interval and under conditions of high ambient scene light intensity to fire the strobe light as a consequence of the time integration of the scene light intensity incident to the photoresponsive element reaching a predetermined value. This fill flash system, however, fails to provide a select ratio between that portion of the exposure which is directly attributable to artificial scene illumination and the remaining portion of the exposure which is directly attributable to ambient scene light illumination over a determinate range of camera-to-subject distances.
More recently, a photographic exposure and flash fire control system as disclosed in U.S. Pat. No. 4,192,587, entitled "Proportional Fill Flash", by Arthur G. LaRocque et al., issued Mar. 11, 1980, in common assignment herewith provided for the firing of a strobe in a fill flash mode of operation in a manner whereby a select ratio may be maintained between that proportion of the exposure which is directly attributable to the artificial scene illumination and the remaining proportion of the exposure which is directly attributable to the ambient scene light illumination. Such a select proportional fill flash is accomplished by providing means whereby the light integration control circuit discounts the artificial illumination provided by the strobe. Toward this end, the system includes a sample and hold circuit which operates to sample the output voltage from a photoresponsive element and to transfer the sampled output voltage directly to a light integrating circuit. However, upon the firing of the strobe, the sample and hold circuit operates to hold the sampled voltage immediately prior to the firing of the strobe and to apply the sampled voltage to the light integrating circuit for the duration of the strobe pulse. In addition, a pre-exposure ambient scene light measurement is made to determine whether the exposure is to be made in a normal flash mode of operation under conditions of ambient scene light intensity below a select level or under the fill flash mode of operation under conditions of high ambient scene light intensity above the select level. Since the electronic flash is fired at the appropriate follow focus aperture corresponding to the subject distance from the camera in the aforementioned manner, provision is made to time the firing of the flash to occur at a slightly larger follow focus aperture under conditions of low ambient scene light intensity below the selected level of ambient scene light intensity measured prior to exposure. Although selecting an exact level of ambient scene light intensity below which the flash should be timed to fire at a maximum follow focus aperture in the ordinary flash mode of operation and above which the flash should be timed to fire at a minimum follow focus aperture in the fill-in flash mode of operation may provide generally satisfactory results under most conditions, it may still be desirable under certain conditions to more precisely control the follow focus aperture at which the flash is fired.
Therefore, it is a primary object of this invention to provide a photographic exposure control system wherein the follow focus aperture at which the flash is fired for a determinate camera-to-subject distance progressively varies between minimum and maximum values in inverse corresponding relation with respect to the progressive variation in the ambient scene light intensity.
It is a further object of this invention to provide a photographic exposure control system wherein the flash is timed to fire at a maximum follow focus aperture for a determinate camera-to-subject distance under conditions of low ambient scene light intensity, a minimum follow focus value under conditions of high ambient scene light intensity, and at appropriate intermediate follow focus aperture values which progressively vary between the minimum and maximum values in inverse corresponding relation with respect to the progressive variation in the ambient scene light intensity between its high and low levels.
Other objects of the invention will be in part obvious and will in part appear hereinafter. The invention accordingly comprises a mechanism and method possessing a construction, combination of elements and arrangement of parts which are exemplified in the following detailed disclosure.